Preparation of two-side coated photosensitive papers

ABSTRACT

PHOTOSENSITIVE PAPERS SUCH AS PROOFING PAPER, IMAGEABLE ON BOTH SIDES, ARE PRODUCED BY (A) FIRST COATING ONE SIDE WITH A COMPOSITION COMPRISING A VOLATILE ORGANIC SOLVENT AND A FILM-FORMING POLYMERIC BINDER, AND PHOTOSENSITIVE INGREDIENTS, FOLLOWED BY DRYING TO EVAPOATE THE SOLVENT AT A CONTROLLED RATE, THEREBY FORMING A FILM, THEN (B) SEQUENTIALLY COATING THE SECOND SIDE WITH THE COMPOSITION IN TWO OR MORE COATING AND DRYING STAGES. CAREFUL CONTROL OF THE MULTI-STAGE COATING CONDITIONS, DRYING STEPS, VOLATILE ORGANIC SOLVENTS, POLYMERIC BINDERS, PHOTOSENSITIVE COMPOSITION, AND PAPER SUBSTRATES YIELD BLISTER-FREE, TWO-SIDE COATED, PHOTOSENSTITVE PAPERS.

United States Patent O 3,674,534 PREPARATION OF TWO-SIDE COATED PHOTOSENSITIVE PAPERS Michael John Connair, Wilmington, Del., assignor to E. I. du Pout de Nemours and Company, Wilmington, Del. No Drawing. Filed Nov. 30, 1970, Ser. No. 93,891

Int. Cl. B44d 1/02, N14 US. Cl. 117-34 5 Claims ABSTRACT OF THE DISCLOSURE Photosensitive papers such as proofing paper, imageable on both sides, are produced by (a) first coating one side with a composition comprising a volatile organic solvent and a film-forming polymeric binder, and photosensitive ingredients, followed by drying to evaporate the solvent at a controlled rate, thereby forming a film, then (b) sequentially coating the second side with the composition in two or more coating and drying stages. Careful control of the multi-stage coating conditions, drying steps, volatile organic solvents, polymeric binders, photosensitive composition, and paper substrates yield blister-free, two-side coated, photosensitive papers.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to a multi-stage process for preparing blister-free paper coated on both sides with filmforming photosensitive compositions.

(2) Description of the prior art Photosensitive formulations are conveniently applied to paper substrates as solutions in volatile organic solvents. They may be thus continuously coated on roll papers utilizing such typical devices for continuously laying down wet films as nip fed three roll reverse roll coating heads, gravure coaters, trailing blade coaters, and Mayer bar coating heads wherein the coating thickness is controlled by a threaded or a wire Wound bar. The wet thickness is adjusted such that the dry thickness after solvent removal is in the desired range of about 0.1 to 1.5 mil. After the solvent has been removed, such as by radiant heating of forced air evaporation, the coated paper is wound up on rolls in the usual way.

For many photoproofing applications it is highly desirable to have paper coated on both sides with the photo sensitive composition. Attempts to coat both sides by the conventional manner described above, with photosensitive compositions dissolved in volatile organic solvents, have been largely unsuccessful. The resulting two-side coated papers have been satisfactorily coated on the first side, but the second side contained numerous blemishes, descriptively termed blisters. Blistered photosensitive paper is unacceptable to the trade primarily because of its appearance and sensitometric behavior.

Elimination of blisters has been sought by various experiments, for example, by altering drying speed, by altering active ingredient content of coating formulations, and by changing coating speeds. While some of these alterations give improved results, they are not entirely practicable for commercial production.

The blister problem is believed to arise from the use of a volatile organic solvent, such as acetone, for the photosensitive compositions applied, coupled with presence of film-forming ingredients, such as polymeric binders, and the necessity to coat and dry the formulations at practical concentrations and rates. Presumably solvent 3,674,534 Patented July 4, 1972 from the second coating penetrates into the substrate until it reaches the first side coating which acts as a barrier. The solvent then returns to the second coating surface where it bubbles through the skin which has been formed by solvent evaporation.

It is an object of this invention to provide a method for preparing blister-free photosensitive papers coated on both sides with photosensitive compositions.

SUMMARY OF THE INVENTION In summary, this invention is directed to blister-free photosensitive paper, comprising:

(a) Essentially opaque paper less than 10 mil thick, having a solvent holdout value of no more than 70, and a coating on both sides of (b) A photosensitive formulation comprising film-forming polymeric binder and a photosensitive component, said coating being in the form of a film not readily penetrated by volatile organic solvents, said film being present on each side of the paper in an amount ranging from 6 to 15 pounds per 3000 square feet of paper; said coating being applied by 1) Applying to the first side of the paper a solution in a volatile organic solvent of a photosensitive formulation comprising a film-forming polymeric binder and a photosensitive component in an amount sufiicient to provide a dry film coating of 6 to 15 pounds per 3000 square feet of paper;

(2) Drying the first-side coat by evaporating the solvent at a rate effective to yield a dry, blister-free film, then (3) Applying to the second side of the paper the solution of step (1) in multiple stages and in an amount sufiicient to provide a dry film of a weight substantially equal to the first side coat, with the proviso that in each stage there is applied no more than 60 percent and no less than 10 percent by weight of the amount applied in step (1), the coating of each second-side application being dried, before application of another coating, by evaporating the solvent at a rate effective to yield a dry, blister-free film.

DESCRIPTION OF THE INVENTION This invention is directed to a process for applying photosensitive formulations, comprised principally of a volatile organic solvent, a photosensitive component and a polymeric binder, to both sides of paper which is readily permeable by said solvent and less than 10 mils thick. The process is effective with conventional coating and drying machinery, in spite of the fact that the first side coat is sufiiciently thick to be substantially impenetrable, or only slowly penetrable, by said solvent. The second-side coat is applied sequentially in steps, under controlled drying conditions.

(1) Paper substrate Since the object of this invention is to prepare blisterfree, two-side coated, photosensitive papers, it should first be noted that the paper used must be essentially opaque to ultraviolet and visible light. This is critical for twoside coated papers.

It is also important that the volatile organic solvent permeability of the paper to be coated be high, and that the paper be thin. Thick paper (10 mil or thicker) and paper with a high solvent holdout can be easily coated on both sides by the process disclosed in copending application Ser. No. 93,890 filed of even date herewith.

Resistance to solvent penetration, or solvent holdout as used herein, is measured by the Schweitzer test which defines the holdout as the time required for a dye solution to penetrate through a sheet of the paper being tested.

The Schweitzer test measures the time, in seconds, necessary for a standard dye solution to sufiiciently penetrate through a sheet of paper to reduce the reflectance value from the paper by 20 percent. To be more specific, a sheet of paper is laid horizontally over the aperture leading to photocells and apparatus for measuring reflectance. The dye solution is applied to the top of the paper; on penetrating the paper, the dye eventually reaches the backside, the side exposed to the reflectometer. The dye absorbs light and thus reduces reflectance. Arbitrarily, a 20 percent reduction in reflectance is chosen for convenience in measurement. The standard dye solution contains 0.2 gram Naphthol Green B (Colour Index 10020, Acid Green 1) dissolved in 30 ml. water and 100 ml. 95 percent ethanol.

The relationship of coating weight and solvent holdout, on a. series of commercial papers supplied by the P. I. Schweitzer, Div. of Kimberly-Clark Corp., is shown in the following table, when coated by a single pass per side with the formulations of Example 7, Belgian Pat. 716,03 7, also disclosed in US. Pat. 3,390,994.

Holdcut value provided by P. J Schweitzer Paper Company. B Coating weight was standard pounds per 3,000 it. per side applied single pass per side. Blisters were seen on the second pass.

It is very desirable, with the formulations described herein, to apply the coatings to both sides of paper in the range of 6-15 pounds per 3000 square feet. At this weight, as is obvious from Table 1, many commercial papers would yield a blistered product when applied single pass per side. The common commercial papers which exhibited this defect include 25 pounds per, 32 pounds per and 42 pounds per 2000 square feet.

It is of course recognized that alternative methods are available and applicable for determining the permeability of paper. One need only establish the correlation between such test results and the results obtained from the Schweitzer test. One such method is the TAP'PI test method T-460 OS68 which measures paper porsity by determining the air resistance of the paper. Test results of the TAFPI test are usually expressed as a Gurley numher, and the higher the Gurley number the more impervious is the paper to passage of air.

(2) Solvents Suitable solvents are those inert toward the photo sensitive composition, for example a leuco dye or salt thereof, a photo-oxidant, and a binder. The solvents are conveniently employed to dissolve these components, mix them, and provide fluid homogeneous media for application to substrates. In general, a solvent should dissolve all components, but be inert to them. It should also be inert to and wet, but not dissolve, the paper to which the photosensitive materials are applied.

To obtain the final coating, the solvent is removed by evaporation. Removal should be sufiicient to yield a coating dry to the touch and the term dry as used herein will be used to mean dry to the touch.

Volatile organic solvents suitable for use in this invention include those boiling in the range of 40-120 C. Representative of such solvents are methanol, ethanol, acetone, l-propanol, 2-propanol, n-butanol, methyl acetate, ethyl acetate, benzene, toluene, methyl ethyl ketone, 3-pentanone, methylene chloride, chloroform, 1,1,2-trichloroethane, tetrahydrofuran, dioxane, and mixtures thereof in various proportions as may be required to dissolve the various components selected for use in the composition. It is particularly desirable that the solvents be volatile enough to be readily removed with low expenditure of thermal energy. Preferred solvents are those boiling between 55 and C.

The volatile organic solvent is the major ingredient of the photosensitive formulations applied by the process of this invention. Usually, the volatile organic solvent comprises 65-90% by weight of the photosensitive formulation to be applied to the paper surface. Preferably the solvent comprises 75-85% by weight of the formulation.

(3) Binders Polymeric binders are present in light-sensitive compositions to thicken and adhere them to the paper substrate and to serve as a matrix for the color-forming photosensitive components. Light-transparent and film-forming polymers are preferred. Representative of suitable polymers are ethyl cellulose, polyvinyl alcohol, polyvinyl chloride, polystyrene, polyvinyl acetate, poly(methyl methacrylate), cellulose acetate, cellulose butyrate, cellulose acetate butyrate, cellulose nitrate, chlorinated rubber, copolymers of the above vinyl monomers, and gelatin. Binder or matrix amounts vary from about 1 part to about 200 parts, preferably 3 to 15 parts, by weight per part of combined weight of the photosensitive components.

When two-side coating some of the preferred compositions exemplified hereinafter to a dry coating weight of from 7 to 12 pounds per 3000 square feet, the percentage of binder in the initially applied solution is normally 8 to 25 percent by weight.

(4) Plasticizer With certain compositions it may be desirable to add a plasticizer to give flexibility to the film or coating. Plasticizers include the polyethylene glycols such as the commercially available carbo-waxes, and related materials, such as substituted phenol-ethylene oxide adducts, for example, the polyethers obtained from o, mand p-cresol, o-, mand p-phenylphenol and p-nonylphenol, including commercially available materials such as the lgepal alkyl phenoxy polyoxyethylene ethanols. Other suitable plasticizers are the acetates, propionates, butyrates and other carboxylate esters of ethylene glycol, diethylene glycol, glycerol, pentaerythritol and other polyhydric alcohols, and alkyl phthalates and phosphates such as dimethyl phthalate, diethyl phthalate, dioctyl phthalate, tributyl posphate, trihexyl phosphate, trioctyl phosphate, triphenyl phosphate, tricresyl phosphate and cresyl diphenyl phosphate. The quantity of optionally added plasticizer is usually not more than 10%, by weight of the binder present.

(5) Photosensitive components The other ingredients of the photosensitive formulations are known in the art. These components are not critical and may vary widely provided they are soluble in the solvent, are non-volatile and do not interfere with the coating operation. Some preferred photosensitive compositions are described in U.S. Pat. 3,445,234, U.S. Pat. 3,390,996, US. Pat. 3,390,994, US Pat. 3,585,038, US. Pat. 3,598,992, and application Ser. No. 99,512, filed Dec. 18, 1970.

It is probable that the plasticizer and photosensitive components of these compositions contribute at least to a minor degree to forming volatile organic solvent-resistant films. The polymeric binder, however, appears to be the major and critical component leading to blisters on twoside coating prior to this invention.

(6) Coating methods (A) General.Except for the multi-pass, or supercoating, aspect of the invention, well-known, conventional, continuous coating apparatus and technology are used. One method in current use is a continuous one in which paper or film from a storage roll is continuously passed by a coating head which applies the formulation, passed under a blown down-flowing current of warm air to evaporate the major part of the volatile organic solvent in the formulation, passed through a zone containing a series of overhead Cal-rod electrical heaters and provided with an upward sweep of heated air to remove residual volatile solvent as a vapor from the formulation, and finally passed to storage on a collecting roll.

Other suitable conventional coating methods include gravure, Mayer bar, reverse roll and knife-over-roll.

Drying can be effected by a blown down flowing current of warm air in the initial drying stage and with a downward rather than upward sweep of air in later drying stages, the air flow in the later stages being controlled by spaced air slots.

(B) Multi-stage.-Photosensitive formulations of from 10 to 40 percent solids are applied to the first side of paper with a coating machine such as, but not limited to, 3 or 4 roll reverse roll, Mayer bar, or blade coater at web speeds of from 4 to 1500 feet per minute. The dry weight of the coating can range from 5 to 15 pounds per 3000 square feet and preferably ranges from 7 to 12 pounds per 3000 square feet.

The firstside coating is then dried, for example, in a hot air impingement dryer. Other types of drying such as radiant heat, microwave and others can also be used.

After the coating has been applied and dried on the first side, the web is reversed and a coating of from to 60 percent by weight, preferably 25 to 45 percent, of that applied to the first side, is applied to the second side by the same technique used to apply the coating to the first side normally from 2 to 5 pounds per 3000 square feet. This process is then repeated until the second side coat has substantially the same coating weight, as that originally applied to the first side. By this technique of sequential multistage coating, and with the above carefully defined conditions, blister-free two-side coated paper are obtained.

The second side requires at least 2 and up to 4 or more lighter-weight coatings to bring the total dry coating weight up to that of the first side. This can range from 5 to 15 pounds per 3000 square feet depending on the intended use of the product, but is usually in the 7 to 12 pounds per 3000 square feet level.

In all instances, coatings on the second side must be dried as gently as the coating on the first side or small bubbles and even fairly large blisters will appear in the coating. Also, in most instances, it has been advantageous to reduce the coating speed at least somewhat, perhaps to percent, when coating the second side.

The weight that can be applied in one pass depends largely on the solvent-hold-out characteristics of the substrate used. The poorer the solvent hold-out, the lighter the coating weight per pass. The coating weight that can be applied is also directly proportional to the percent solids in the coating lacquer. As an example, for Schweitzer 32 pound HG paper which has fairly low solvent hold-out, using the formulation of Example 7 of Belgian Pat. 716,- 037 and at 17 to 18 percent solids, the maximum coating that can be applied as the first pass on the second side is in the range of 3 or 4 pounds per 3000 square feet, while at 20 percent solids the maximum coating is in the range of 5 to 6 pounds.

These and other embodiments of this invention are illustrated in greater detail in the following example.

EXAMPLE A typical photosensitive composition was prepared containing the following ingredients:

Grams 2,2'-bis(o-chlorophenyl)-4,4,5,5'-tetrakis(m methoxyphenyl)biimidazole 0.4180 Tris(4-diethylamino-o-tolyl)methane 0.0900 p-Toluenesulfonic acid monohydrate 0.400 Trimethyl 3,3,3"-nitrilotripropionate1.0 ml. 9,10-phenanthrenequinone 0.054

Cellulose acetate butyrate (Eastman Chemicals EAB-17140) 6.0 Polyethyleneoxide adduct of o-phenylphenol (2.25 moles of ethylene oxide per mole of the phenol, average formula C H C H O (CH CH O) H) 3 .0

The above formulation as a 25 percent solids solution in :10 acetonezZ-propanol mixture was coated on Schweitzer HG paper (32 pounds per 2000 square feet). The felt side was coated first to yield a dry coating weight of 9.6 pounds per 3000 square feet. Slow, careful drying by solvent evaporation was effected in a two-zone dryer with the first zone at 150 F. and the second zone at 275 F.

The wire side was then coated. A first coat was applied to yield a dry weight of 5.4 pounds per 3000 square feet. After drying as above, a second coat, dry weight of 2.9 pounds per 3000 square feet, was applied to the 'wire side and also dried as above. In this manner, blister-free, two-side coated photosensitive paper was obtained.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of applying a blister-free photosensitive coating to both sides of a paper, said paper being essentially opaque to actinic radiation, less than 10 mils thick, and having a Schweitzer solvent holdout value of no more than about 70, comprising the steps of (l) applying to the first side of the paper, in an amount sufficient to provide a dry film coating of 6 to 15 pounds per 3000 square feet of paper, a solution of a photosensitive formulation dissolved in a volatile organic solvent having a boiling point between 40 and C., said photosensitive formulation comprising a photosensitive component and a film-forming polymeric binder, said polymeric binder being one which forms on drying a film substantially impenetrable by said solvent;

(2) drying the first-side coat by evaporating the solvent at a rate effective to yield a dry, blister-free film; then 3) applying to the second side of the paper, in multiple passes and in an amount sufficient to provide a dry film of a weight substantially equal to the firstside coat, a solution of step (1), with the proviso that in each stage there is applied no more than 60 percent by weight and no less than 10 percent by weight of the amount applied in step (1 the coating of each second-side application being dried before application of another coating by evaporating the solvent at a rate effective to yield a dry, blister-free film.

2. The method of claim 1 wherein the paper is 3 to 5 mils thick.

3. The method of claim 1 wherein from 25 percent by weight to 45 percent by Weight of amount of coating applied in step (1) is applied in each pass of step (3).

4. The method of claim 1 wherein the solvent has a boiling point between 55 C. and 85 C.

5. Blister-free photosensitive paper comprising (a) paper essentially opaque to actinic radiation, 3 to 5 mils thick, having a Schweitzer solvent holdout E References Cited BO more than 70, and a coating 011 both UNITED ST P (b) a photosensitive formulation comprising a film- 33??? 432 g g at T 117' 68 forming polymeric binder and a photosensitive com- 5 g g 9 et a ponent, said coating being in the form of a film not onant n 1 readily penetrated by volatile organic solvents, said NORMAN TORCHIN Primary Examiner fil b t h d of the er'n an m 6mg pres m e Pap 1 I. WINKELMAN, Assistant Examiner amount ranging from 6 to 15 pounds per 3000 square 10 feet of paper; and said coating having been applied US by the process of claim 1. 117 68 152, 155 

